4.1 Digital Line Coding
Digital Line Coding is a special coding system chosen to allow transmission to take place in a communications system. The chosen code or pattern of voltage used to represent binary digits on a transmission medium is called line encoding.
The types of line encoding are polar, unipolar and bipolar.
The polarity of a code can be positive or negative or in certain cases an idle line can be incorporated into the code.
Digital line encoding takes place at the physical layer of the OSI model.
Unipolar encoding only makes use of one polarity, i.e. positive voltage, to represent a binary 1 and it uses an idle line to represent a binary 0, i.e. zero voltage.
Unipolar line coding is a very basic method of encoding and is rarely used with most line encoding being either polar or bipolar or variations of these.
Polar encoding represents bits by using two levels of polarity or amplitude: positive and negative.
The most common type is polar encoding is Non Return to Zero (NRZ).
A variation on polar encoding is biphase encoding where although positive and negative polarity is still used the code works in the middle of the signal. This has the advantage of making this type of code self clocking. An example of biphase line encoding is Manchester encoding.
Next: 4.2 Non Return to Zero